Page:Popular Science Monthly Volume 69.djvu/123

Rh of the mechanical vibrations. We know that a magnet subjected to strain undergoes changes in its magnetization and so the question arises whether the earth's magnetized rocks may not likewise give some indication of their state of strain during an earthquake by slight magnetic fluctuations. Or, an earthquake may be accompanied by a redistribution of the magnetic rocks or of the electric earth-currents known to exist, and thus give rise to a possible magnetic effect.

Enough has been said to show that a careful and exhaustive investigation of seismic effects recorded on magnetographs is certainly one that merits undertaking. The Department of Terrestrial Magnetism of the Carnegie Institution of Washington, in connection with the study of the magnetic effect, recorded simultaneously over the entire globe coincident with the outbreak of Mt. Pelé, on May 8, 1902, is making a systematic study of the volcanic and seismic effects recorded by magnetic instruments with the.cooperation of the Coast and Geodetic Survey and of the Canadian Meteorological Service. A paper by Mr. J. E. Burbank, published in Vol. X., p. 113, of the journal, Terrestrial Magnetism, brought the investigation up to the time of the installation of the seismographs at the Coast and Geodetic Survey Observatories two to three years ago; a second paper, to be published in the course of the year, will continue the research as based upon seismic and magnetic instruments in operation at the same observatory.

It had been noticed for some time that magnetic instruments responded to certain earthquakes, but the cases noted were of such a class as to convey the first impression at once that the effects recorded were mechanical ones. Milne in 1898 made quite an exhaustive investigation of this class of effects for the whole earth and covering the period from 1889 to 1897. He likewise found that these effects were not invariably recorded at every magnetic observatory. He considered the results inconclusive and deemed it necessary to await the time when both seismograph and magnetograph records could be had at the same place. A recent notable contribution to the subject based on magnetic records at one observatory, without, however, at the same time corresponding-seismological data, has been made by Dr. Messerschmitt, in charge of the Munich Magnetic Observatory.

Previous magneticians, such as Eschenhagen, Wild and Liznar, had found that from a comparison of the effects recorded on magnetographs at various European observatories the effects, in certain notable earthquakes, progressed from station to station with the velocity of about three kilometers i. e. the rate of propagation of the long or surface seismic waves. This measurable difference in time between any two stations and its correspondence with the time interval required for the transmission of the surface waves was a very good indication that a purely mechanical effect had been recorded and not a distant magnetic